Identification of conserved isotype-defining variable region sequences for four vertebrate beta tubulin polypeptide classes.

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Abstract
We report the determination of the complete sequences for two chicken .beta. tubulin genes, .beta.3 and .beta.5. Taken with the previously published efforts, we have determined the primary structures of five of the seven .beta. tubulin genes in this vertebrate species. A comparison of these sequences unambiguously reveals that amino acid sequence variations among different .beta. tubulin gene products are distinctly clustered within an otherwise highly conserved framework of the .beta. tubulin molecule. To determine the extent to which this pattern of structural heterogeneity is conserved among vertebrates, we have isolated novel .beta. tubulin sequences from human and mouse cDNA libraries and compared these and all other known vertebrate .beta. tubulin sequences with the family of chicken polypeptide sequence. What emerges from such comparison is the recognition of distinct, evolutionarily conserved isotypes of .beta. tubulin that are distinguished primarily by their characteristic carboxyl-terminal variable region sequence, and, to a lesser extent, by sequence in an amino-terminal variable domain as well. These correlations represent a convincing demonstration that multiple .beta. tubulin genes in vertebrates encode a family of closely related but structurally distinct .beta. tubulin isotypes and further serve to define the sequences of four classes of polypeptide isotypes that constitute that family.